title crack extension in hydraulic fracturing of shale
TRANSCRIPT
Title Crack Extension in Hydraulic Fracturing of Shale Cores UsingViscous Oil, Water, and Liquid Carbon Dioxide
Author(s)Bennour, Ziad; Ishida, Tsuyoshi; Nagaya, Yuya; Chen,Youqing; Nara, Yoshitaka; Chen, Qu; Sekine, Kotaro; Nagano,Yu
Citation Rock Mechanics and Rock Engineering (2015), 48(4): 1463-1473
Issue Date 2015-06-12
URL http://hdl.handle.net/2433/201552
Right
The final publication is available at Springer viahttp://dx.doi.org/10.1007/s00603-015-0774-2.; The full-textfile will be made open to the public on 12 June 2016 inaccordance with publisher's 'Terms and Conditions for Self-Archiving'.
Type Journal Article
Textversion author
Kyoto University
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Crack Extension in Hydraulic Fracturing of Shale Cores Using Viscous Oil, Water, and
Liquid Carbon Dioxide.
Ziad Bennour1, Tsuyoshi Ishida*1, Yuya Nagaya1, Youqing Chen2, Yoshitaka Nara1,5, Qu
Chen3, Kotaro Sekine4 and Yu Nagano4
1Department of Civil and Earth Resources Engineering, Graduate School of
Engineering, Kyoto University, Kyoto, Japan
2Department of Energy Science and Technology, Graduate School of Energy Science,
Kyoto University, Kyoto, Japan
33D Geoscience Inc., Tokyo, Japan
4Japan Oil, Gas and Metals National Corporation, Chiba, Japan
5Now he is working for Tottori University, Tottori, Japan
*Corresponding author: Tsuyoshi Ishida
Department of Civil and Earth Resources Engineering, Graduate School of Engineering,
Kyoto University, Kyoto Daigaku Katsura, Nishikyo-ku, Kyoto 615-8540, Japan
E-mail: [email protected]
Tel.: +81 75 383 3209; Fax: +81 75 383 3213
1
Abstract
We performed hydraulic fracturing experiments on cylindrical cores of anisotropic
shale obtained by drilling normal to the sedimentary plane. Experiments were
conducted under ambient condition and uniaxial stresses, using 3 types of fracturing
fluid: viscous oil, water, and liquid carbon dioxide (L-CO2). In the experiments using
water and oil, cracks extended along the loading direction normal to the sedimentary
plane under the uniaxial loading and extended along the sedimentary plane without
loading. These results suggest that the direction of crack extension is strongly affected
by in situ stress conditions. Fluorescent microscopy revealed that hydraulic fracturing
with viscous oil produced linear cracks with few branches, whereas that with water
produced cracks with many branches inclining from the loading axis. Statistical analysis
of P-wave polarity of acoustic emission waveforms showed that viscous oil tended to
induce Mode I fracture, whereas both water and L-CO2 tended to induce Mode II
fracture. Crack extension upon injection of L-CO2 was independent of loading condition
unlike extension for the other two fluids. This result seemed attributable to the low
viscosity of L-CO2 and was consistent with previous observations for granite specimens
that low-viscosity fluids like CO2 tend to induce widely extending cracks with many
branches and with Mode II fractures being dominant. These features are more
advantageous for shale gas production than those induced by injection of conventional
slick water.
2
Keywords: Anisotropy; Acoustic emissions; Shale; Hydraulic fracturing; Carbon
dioxide; Viscosity
3
1. Introduction
Shale gas refers to the natural gas trapped within formations of shale, a fine-
grained sedimentary rock that can be a rich source of petroleum and natural gas.
However, since the low permeability of shale greatly inhibits the flow of gas from
reservoir rocks to production wells, the economic viability of developing shale gas
depends on effective stimulation of reservoirs. Recently, horizontal wells employing a
multistage hydraulic fracturing technique have become the stimulation method of choice
and proved successful in shale gas reservoirs (Arthur et al., 2008).
Hydraulic fracturing is the process of initiation and propagation of cracks by
injection of a fluid at a pressure higher than the failure stress of the rock. The technique
of hydraulic fracturing stress measurements was developed by Hubbert and Willis
(1957). Building on this work, Cleary (1958) presented an early version of hydraulic
fracturing theory and the possibility of fracture control. Since then, many theoretical and
experimental investigations have been conducted (i.e. Zoback et al., 1977; Schmitt and
Zoback, 1989; Haimson and Cornet, 2003).
Underground formations are subjected to complex stress fields and affected by
various geological processes, as described by Amadei and Stephansson (1977) and Zang
and Stephansson (2010). The magnitude and direction of the principal stresses are
important in hydraulic fracturing because they control the amount of pressure required
4
to create and propagate a crack, the direction of the crack and the crack shape. The
direction that a crack induced by hydraulic fracturing takes in a rock is also a function
of several variables, including anisotropy of rock strength. Sun et al. (2011) investigated
crack extension during hydraulic fracturing in oil shale and found that the cracks are
elliptical and that cracks extend along different directions due to anisotropic properties
and the in situ stress condition.
Ishida et al. (2004; 2012; 2013) have previously conducted hydraulic fracturing
experiments using carbon dioxide (CO2), water, and viscous oil and found that low
viscosity fluids such as CO2 tend to induce widely extending cracks with many
branches. These cracks should be better suited for producing shale gas because they
have a larger surface area within the shale than those induced by water.
Accordingly, by monitoring acoustic emissions (AE), this study focuses on the
crack growth in shale cores in relation to the effects of stress condition and anisotropy
due to sedimentary planes. This study also examines the effect of the viscosity of
various fracturing fluids (viscous oil, water, and CO2) on the fracture mode and on the
microscopic features of the induced cracks. The results of this study may be beneficial
in industrial energy projects such as extraction of unconventional gas from shale and the
sequestration of greenhouse gases.
5
2. Experimental methodology
2.1. Shale cores
Experiments were performed on cylindrical shale cores measuring roughly 170
mm in length and 85 mm in diameter. All samples were retrieved by drilling blocks
obtained from a depth of 275 m below sea level while mining a drift at the Kushiro coal
mine in Hokkaido, Japan. The blocks were sandy shale from Palaeogene Harutori coal
bearing formation. Because sedimentary planes were clearly observed on the surfaces of
the blocks, the drilling direction was selected to be normal to the sedimentary plane.
A hole for hydraulic fracturing with a diameter of 10 mm was drilled in the side of
the core at the vertical midpoint. A Cartesian coordinate system was applied to the
cores: the Z-axis was set along the cylindrical axis and normal to the sedimentary plane,
the X-axis was set along the drilled hole, and the Y-axis was set orthogonal to the other
2 axes, as shown in Fig. 1.
P-wave velocities were measured along the three selected axes for each specimen.
The results of the P-wave velocity measurements clearly show orthogonal anisotropy
due to the sedimentary bedding with the exceptions of those in the cores K-01 and Kc-
12 (Table 1).
2.2. Uniaxial stress conditions
Hydraulic fracturing tests were run at ambient condition and two uniaxial stresses
of 1 and 3 MPa, applied along the Z-axis (σz, normal to the sedimentary plane). Four
6
10-mm-long strain gauges were attached to the cores to confirm that loading was
uniformly applied to the cores before testing.
2.3. Fracturing fluid and pressurizing system for hydraulic fracturing
Three types of fluids, namely, viscous oil, water, and liquid carbon dioxide (L-
CO2), were used for hydraulic fracturing. The viscous oil used was automobile
transmission oil (Mahha Super Transmission Oil 75W-90; Fuji Kosan Co. Ltd., Tokyo,
Japan). At the experimental temperature, the viscous oil was 270 times more viscous
than water, whereas the L-CO2 has about 1/10 the viscosity of water.
A packer with a pressurizing section of 30 mm was set in the center along hole.
Fluids were injected at a constant flow rate of 1 ml/min, and injection pressure was
recorded at 0.1 s intervals. A syringe pump with controllable discharge flow rate was
connected to the hole via connecter pipes and used to inject the fluid. A pressure
transducer was placed at a position close to the packer to detect the actual fluid pressure
of the injection.
2.4. AE monitoring system
An array of 16 PICO sensors (Physical Acoustics Corporation), which were
cylindrical, 3 mm in diameter, and 4 mm in length, with a resonance frequency of
around 400 kHz, was used to record AE events. The detected AE signals were amplified
by a total of 84 dB (36 dB in the pre-amplifier and 48 dB in the signal conditioner,
7
except in experiments that used L-CO2 as fracturing fluid, where signal conditioner was
set to 24 dB due to a high level of noise), then recorded on a hard disk via an A/D
converter. For each event, the record length and sampling time of the A/D converter
were selected to be 2048 words (each word size of 32 decimal digits) and 0.1 μs,
respectively, with 12-bit resolution. The dead time was set to be 1 ms after recording an
event to prevent the hard disk from recording too much noise due to the vibrations
following a large AE event. Recording of an AE event was triggered when one of the
signals from the 16 sensors exceeded 3 V, and the triggered events usually have more
than a few signals.
The AE events were monitored and recorded during the injection. Later in the
process, the AE hypocenters were located by P-wave arrival time at sensors based on a
least squares method and then compared to the results of visual detection in order to
trace the shape and orientation of cracks induced on the surface of the cores.
Furthermore, the mode of fracturing was examined by the statistical analysis
determining the polarities of P-wave initial motion along their respective AE waveforms
following Zang et al. (1998) and Graham et al. (2010). Then, depending on their
polarity, they were categorized as either compression or dilatation. The number of
compressional initial motions and the number of dilatational were counted for each AE
event. A ratio of the compressional initial motions to the total number of the
compressional and the dilatational initial motions could then be obtained.
8
2.5. Microscopic observation of the cracks
Fluorescence microscopy (Nishiyama and Kusuda, 1994) can be used to visualize
cracks and pores in rocks using resin mixed with a fluorescent substance. In the present
study, microscopy was performed on the shale samples fractured by injection of viscous
oil and water.
To prepare samples for observation, the cores were soaked in the resin for about a
week under vacuum and then heat-treated at up to 90°C to solidify the resin. Later, cores
were cut into sections normal to the crack plane extension induced by hydraulic
fracturing and were prepared for observation on a microscope under ultraviolet light.
9
3. Results
The experiments conducted on the shale cores by testing each of the three
hydraulic fracturing fluids under an ambient condition and two uniaxial stresses of 1
and 3 MPa. The tests were repeated to confirm the tendency of the results, thus yielding
a total of 12 tests. Table 2 summarizes the test results where the last column shows the
orientation of the induced cracks in relation with the stress conditions and the used
fracturing fluid.
In this section, the results of typical one case for the respective fracturing fluid
and loading conditions are shown. The selection of the typical case was made solely on
the numbers and quality of the located AE sources to compare the observed surface
cracks. The located AE sources are only those were recorded 1 s before to 15 s after the
breakdown.
3.1. Testing observations without loading
In the experiments of the cores K-01 and K-03, water was used as the fracturing
fluid, without loading (0 MPa stress along the Z-axis). Figure 2 shows an example of
pressure profile in the hydraulic fracturing test of K-03, the pressure built-up and
breakdown occurred at 8.62 MPa. In both of the cores, the crack extended horizontally
along a sedimentary plane and was easily traced on the surface of the cores (as in Fig.
3a). The AE events detected by the sensors during the fracturing of the cores were
located three-dimensionally. Figure 3b shows AE sources of the core K-03 projected on
10
the horizontal plane (X-Y plane) and the two vertical planes (X-Z and Y-Z planes). AE
sources seen in the vertical planes are distributed in the horizontal direction. These AE
results are consistent with the crack trace on the surfaces of the cores.
When a more viscous liquid than water (viscous oil, K-05) was injected, the
fracture extended horizontally along the sedimentary bedding, the same propagation as
when the fracturing fluid was water. The aperture of the crack induced was noticeably
much larger and clearer (Fig. 4a) when the fracturing fluid was oil than when it was
water. The projection of the AE source location distribution in Y-Z plane supports the
inference of the fracture path from its surface trace (Fig. 4b).
The least viscous fluid (L-CO2) was injected into core Kc-12 under 0 MPa of
stress along the Z-axis, an intense audible acoustic emission occurred at the moment of
breakdown and the fracture extension was not horizontally parallel to the sedimentary
planes. This result is unlike the observations recorded for the other fluids, for example
the cores K-03 and K-05. Here, the crack extended in a complex inclined direction and a
very large aperture that completely split the core was observed on the surface (Fig. 5a).
Accordingly, the distribution of AE sources locations (Fig. 5b) showed that the fracture
extended at deviated direction of 10 to 30° from the vertical, which is consistent with
the clearly traced fracture on the surface.
3.2. Testing observations under uniaxial loading
11
In the hydraulic fracturing of cores K-02 and K-04 using water as the fracturing
fluid, under uniaxial loading of 1 and 3 MPa along the Z-axis, the crack observed on the
surface of the cores extended along the loading direction, which was normal to the
sedimentary plane. Fig. 6 shows the case of the core K-04 under 3 MPa loading. The
both of the cores indicate that the fracture extension was in the loading direction.
When the other cores (K-07 and K-09) were tested using oil as the fracturing fluid
under 1 and 3 MPa of stress along the Z-axis, the crack again extended vertically,
normal to the sedimentary plane, the same as when the fracturing fluid was water. Fig.
7 shows the case of the core K-09 under 3 MPa loading. The distribution of the AE
sources shows that the fracture extended in the vertical direction (Z-axis) in both the X-
Z and Y-Z planes, which is consistent with the clearly fracture trace on the surface of the
cores.
The last experiments were conducted by injecting L-CO2 in the cores Kc-01,
Kc-06 and Kc-07 under 3 MPa of stress along the Z-axis. The cracks observed on the
surface of the core Kc-01 extended in two horizontal bedding planes with an inclined
crack connecting them though it was under loading. As for the last 2 cores extended in a
nearly vertical, but slightly inclined direction as shown in Fig. 8 which is the case of the
core Kc-07, unlike in the cores injected with water and oil. The distribution of AE
sources locations in Y-Z plane (as in Fig. 8b) shows also that the fracture extended at
deviated direction from the vertical loading direction.
12
3.3. Analysis of mode of fracturing
The effect of the difference in fracturing fluid viscosity on the mode of fracturing
was examined using the cores tested under 3 MPa of uniaxial loading, K-04 (water), K-
09 (oil), and Kc-07 (L-CO2), because the crack extension under loading seemed to be
stable and more clearly affected by fluid viscosity than in the cores without loading, due
to the decreasing effect of the sedimentary planes.
To classify the polarities of the P-wave initial motion arrivals as either
compressional or dilatational, a polarity calibration test was performed by dropping a
steel ball on one side of a steel plate after attaching sensors to the other side. A
compressional wave resulted from the impact and radiated from the point of the impact
to the sensors attached to the opposite side of the plate. Because the upward trace was
recorded in all P-wave initial motion arrivals in this test, the upward trace was
recognized and considered as the compressional motion.
The ratio of the compressional to the total initial motions was obtained for 30
large events each for the viscous oil, water, and L-CO2 injection. The selection of these
events was based on the events having at least 12 out of 16 initial motions on the
sensors. The analysis revealed that the higher ratios at 60–90% for the viscous oil
injection and at 40–60% for both the water and the L-CO2 injections (Fig. 9). The ratio
should be 100% for Mode I fracture (tensile fracture) and should be 50% for Mode II
fracture (in-plane shear fracture). Thus, these results indicate that the injection of
13
viscous oil tended to induce Mode I fracture, whereas the injection of water and of L-
CO2 tended to induce Mode II fracture.
3.4. Microscopic examination of cracks
The opening of the crack in the K-05 sample (oil) is somewhat wider than that in
the K-03 sample (water), as can be seen by comparing Figs. 3a and 4a. However, there
is no marked difference in the fractures between the two samples under loading at 0
MPa, which is probably due to the crack extension running along the sedimentary
planes. Comparing the cracks induced under loading at 3 MPa by water and oil, the core
K-04, which was fractured by water, contains more inclined cracks having an angle
from the loading direction in its path than the core K-09 by oil, (Fig. 10). On the
inclined crack surfaces, shear stress develops with the angle, and it becomes the
maximum at the angle of 45 degrees to the loading axis. Thus, the crack feature having
the inclinations in its path more easily causes shear fracture than that of oil, being
consistent with the tendency of fracture mechanism shown in Figure 9.
14
4. Discussion
We found the following tendency regarding crack extension from the surface
crack and AE source distribution observations. In the experiments that used water or oil
as the fracturing fluid, cracks tended to extend along the loading direction normal to the
sedimentary plane with uniaxial loading and tended to extend along the sedimentary
plane if tests were run without loading. The elastic theory indicates that a crack extends
in the direction of maximum compressive stress. Thus, the direction of crack extension
under uniaxial loading in the present study is consistent with elastic theory. When there
was no loading, the crack extended along the weakest plane due to the sedimentary
bedding, even though elastic theory indicates that the crack can extend in any direction.
These experimental results suggest that the direction of crack extension is strongly
affected by in situ stress condition and rock-strength anisotropy. Taking these findings to
the field application, if shale strata are horizontally situated and horizontal stress is large,
the cracks induced by hydraulic fracturing will tend to extend horizontally. In contrast,
when vertical stress is large relative to horizontal stress, for example in a shale stratum
at great depth, the cracks will tend to extend vertically, normal to sedimentary planes,
and may extend into a different stratum neighboring the shale.
In the experiments using L-CO2 as a fracturing fluid, crack extension shows an
independency of loading condition and the presence of bedding plane unlike in those
using water and oil. In addition, even when the same conditions of loading applied, the
15
crack extension is different among the cores tested by L-CO2. The unstable tendency
independent of the loading condition might be due to the specific properties of L-CO2.
The fracture mode can be identified by examining the polarities of P-wave initial
motions and obtaining a ratio of the compressional initial motions to a total number of
initial motions. The ratio should be 100% for Mode I fracture (tensile fracture) and 50%
for Mode II fracture (in-plane shear fracture). Thus, our results indicate that the viscous
oil injection tends to induce Mode I fracture, whereas both water injection and L-CO2
injection tend to induce Mode II fracture.
In order to quantify the difference caused by different viscosity, a statistical
analysis of the number of branches created by water and oil is carried out. The fractured
samples were cut and the section of 8 mm in length from the injection hole along the
crack path is observed by setting 20 scanning lines of 0.4 mm interval (Fig. 10). By
counting the number of fractures that cross the scanning lines (Fig. 11), the injection of
oil seems to create branches at only few parts of the crack path, while the water
injection creates random number of branches at any point of the crack path. It is
noticeable also that the total number of cracks branches resulted by water injection, 55,
is larger than those resulted by oil injection, 38, although this observation is restricted
only in the section close to the injection hole.
Furthermore, cracks inclining to the loading direction and branching from a main
crack were mainly observed in samples fractured with water. In addition, because shear
16
stress develops on an inclined plane with an angle from the loading direction, the
difference in the crack feature is consistent with the AE monitoring results that Mode II
AE events increased with decrease of the fluid viscosity as shown in Figure 9.
The crack extension in L-CO2 appeared to be unstable even under the same stress
condition. This seems be strongly affected by the difference of fracturing mode due to
viscosity of the fracturing fluid, unlike in other fluids, because the viscosity of L-CO2 is
1/10 and 1/2700 that of water and viscous oil, respectively (see Table 2). Since the slick
nature of L-CO2 makes it easily able to fill even the tiniest spaces, crack extension most
likely becomes very sensitive to even small defects in the core. In addition to the low
viscosity, the phase of CO2 changes from liquid to gas corresponding to the pressure
decrease after crack extension. Since the compressibility of the gas is much larger than
that of the liquid, expansion of CO2 gas may help to connect defects inside of a core and
cause the fracture to extend in an inclined direction, and also result a much larger sound
than that in water or oil injection at the moment of fracture. Ishida et al. (2004; 2012;
2013) previously conducted hydraulic fracturing experiments in granite specimens using
carbon dioxide, water, and viscous oil. These studies revealed that low viscosity fluids
like CO2 tend to induce widely extending cracks with many branches and with the
dominant fracture type being Mode II. The experimental results for the shale cores in
the present study seem to be consistent with the tendency observed in granite specimens,
particularly with the tendency that crack extension with Mode II dominant fractures
17
during CO2 injection is very sensitive to defects, such as tiny spaces in a core. CO2
fracturing, as opposed to conventional slick water fracturing, can likely induce cracks
with better features for shale gas production, for example, a larger surface area. In
addition, there is a possibility for CO2 to be used for enhance shale gas production and
recovery (Kalantari-Dahaghi 2010), because shale's affinity for CO2 is stronger (about
10 times greater) than the affinity for methane (Nuttall 2006) and displacement of
absorbed methane (shale gas) with CO2 is expected. .
Taken together, these and past findings suggest that CO2 fracturing is a promising
new method for shale gas recovery.
18
5. Conclusion
The results of this study are as follows:
(1) Surface crack observations and AE source distributions revealed that cracks
extended along the loading direction normal to the sedimentary plane under uniaxial
loading in the experiments that used water and oil as fracturing fluids. On the other
hand, they extended along the sedimentary plane when tests were run without loading.
These results suggest that the direction of crack extension is strongly affected by in situ
stress condition. However, the L-CO2 fracturing did not show this tendency, which is
probably related to its low viscosity.
(2) The ratio of polarity of P-wave initial motions indicated that injection of viscous oil
tends to induce Mode I fracture and both injection of water and injection of L-CO2 tend
to induce Mode II fracture.
(3) Fluorescence microscopy observation revealed that hydraulic fracturing by viscous
oil injection produces linear cracks with few branches, whereas hydraulic fracturing by
water injection produces cracks with many branches inclining from the loading axis.
This is consistent with the differences in fracture mode based on differences in the
viscosity of the fracturing fluid, as shown above.
(4) The crack extension after fracturing with L-CO2 is unlike those after fracturing with
other fluids, which seems to be due to the low viscosity of L-CO2. This result seems to
be consistent with the tendency observed by researchers of our group in granite
19
specimens, where low-viscosity fluids like CO2 tend to induce widely extending cracks
having many branches with Mode II fractures being dominant. This pattern of cracking
presents an advantage for shale gas production by fracturing with L-CO2 over fracturing
with conventional slick water.
20
Acknowledgment
The authors wish to acknowledge the kind support of Kushiro Coal Mine Co. Ltd. for
providing the shale blocks that were used to prepare the cores for the experiments.
21
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24
Figure captions
Figure 1 Diagram and photograph of the core dimensions and coordinates.
Figure 2 Test profile for shale core K-03 (water) under 0 MPa loading.
Figure 3 (a) Photograph with magnification of observed crack (red box) on the surface
and (b) AE source locations in core K-03 under 0 MPa loading and injection with water.
Figure 4 (a) Photograph with magnification of observed crack (red box) on the surface
and (b) AE source locations in core K-05 under 0 MPa loading and injection with oil.
Figure 5 (a) Photograph of observed crack (red box) on the surface and (b) AE source
locations in core Kc-12 under 0 MPa loading and injection with L-CO2.
Figure 6 (a) Photograph of observed crack (red box) on the surface and (b) AE source
locations in core K-04 under 3 MPa loading and injection with water.
Figure 7 (a) Photograph of observed crack (red box) on the surface and (b) AE source
locations in core K-09 under 3 MPa loading and injection with oil.
Figure 8 (a) Photograph of observed crack (red box) on the surface and (b) AE source
locations in core Kc-07 under 3 MPa loading and injection with L-CO2.
Figure 9 Polarity ratios of compressional initial motions in 30 AE events for each
hydraulic fracturing experiment using viscous oil, water, and L-CO2.
Figure 10 Fluorescence microscopy of the crack shape after (a) injection of water into
core K-04 and (b) injection of oil into core K-09. In the both photos, the cracks
propagated across the sedimentary planes from the injection hole, which is located in
25
the lower end of the photos. The lines of 0.4 mm interval are scanning lines to count
numbers of crack branches shown in Figure 11.
Figure 11 Comparison between the numbers of crack branches made by water and
viscous oil at the same distance.
26
Table captions
Table 1 Anisotropy of P-wave velocity in shale cores
Table 2 Summary of the main experimental results
27
Table 1 Anisotropy of P-wave velocity in shale cores
P-wave velocity (km/s)
Core X Y Z
K-01 3.23 3.11 3.55
K-03 3.12 3.13 2.75
K-04 3.24 3.24 2.92
K-05 3.65 3.36 2.28
K-07 3.00 3.11 2.84 K-09 2.80 3.50 2.87
Kc-01 3.20 3.30 2.90
Kc-06 3.36 3.19 3.10
Kc-07 3.28 3.28 3.01
Kc-12 3.59 3.73 3.82
Table 2 Summary of the main experimental results
Core Loading stress
σz (MPa) Fracturing fluid
(viscosity, mPa∙s) Injection rate
(ml/min) Breakdown
(MPa)
Located (Detected) AE sources
Fracture orientation
K-01 0.0 water (1) 1.00 5.24 13 (122) Parallel to bedding
K-03 0.0 water (1) 1.00 8.62 32 (72) Parallel to bedding
K-02 1.0 water (1) 1.00 13.08 22 (160) Loading direction
K-04 3.0 water (1) 1.00 16.44 65 (205) Loading direction
K-05 0.0 oil (270) 1.00 9.95 26 (91) Parallel to bedding
K-07 1.0 oil (270) 1.00 8.86 62 (244) Loading direction
K-09 3.0 oil (270) 1.00 8.86 39 (319) Loading direction
Kc-01 3.0 L-CO2 (0.10) 1.00 6.91 36 (336) Different direction
Kc-06 3.0 L-CO2 (0.10) 1.00 6.81 0 (41) Different direction
Kc-07 3.0 L-CO2 (0.10) 1.00 6.08 42 (169) Different direction
Kc-12 0.0 L-CO2 (0.10) 1.00 8.12 55 (245) Different direction
28
Figure 1 Diagram and photograph of the core dimensions and coordinates.
170
mm
85 mm
Y
Hole for injection
Z
Sedimentary planes
σz
X
Figure 2 Test profile for shale core K-03 (water) under 0 MPa loading.
Time (s)
Breakdown pressure 8.62 MPa
Pum
p pr
essu
re (M
Pa)
AE
cou
nt (1
/s)
AE
Fluid pressure
Figure 3 (a) Photograph with magnification of observed crack (red box) on the surface and (b) AE source locations in core K-03 under 0 MPa loading and injection with water.
a b
Figure 4 (a) Photograph with magnification of observed crack (red box) on the surface and (b) AE source locations in core K-05 under 0 MPa loading and injection with oil.
a b
Figure 5 (a) Photograph of observed crack (red box) on the surface and (b) AE source locations in core Kc-12 under 0 MPa loading
and injection with L-CO2.
a b
Figure 6 (a) Photograph of observed crack (red box) on the surface and (b) AE source locations in core K-04 under 3 MPa loading and
injection with water.
a b
Figure 7 (a) Photograph of observed crack (red box) on the surface and (b) AE source locations in core K-09 under 3 MPa loading and
injection with oil.
a b
Figure 8 (a) Photograph of observed crack (red box) on the surface and (b) AE source locations in core Kc-07 under 3 MPa loading
and injection with L-CO2.
a b
Figure 9 Polarity ratios of compressional initial motions in 30 AE events for each hydraulic fracturing experiment using viscous oil,
water, and L-CO2.
Num
ber
of A
E e
vent
s
Ratio of the compressional motions to the total initial motions in an event
Water L-CO2
Oil
1-10 10-20 20-30 30-40 40-50 50-60 60-70 70-80 80-90 90-100
Inclined cracks
b a σz σz
Direction of crack propagation
The injection hole
0.4 mm
8.0 mm
7.6 mm
7.2 mm
6.8 mm
6.4 mm
6.0 mm
5.6 mm
5.2 mm
4.8 mm
4.4 mm
4.0 mm
3.6 mm
3.2 mm
2.8 mm
2.4 mm
2.0 mm
1.6 mm
1.2 mm
0.8 mm
0.0 mm
Figure 10 Fluorescence microscopy of the crack shape after (a) injection of water into core K-04 and (b) injection of oil into core K-09. In the both photos, the cracks propagated across the sedimentary planes from the fracturing hole, which is located in the lower end of the photos. The lines of 0.4 mm interval are scanning lines to count numbers of crack branches shown in Figure 11.
Figure 11 Comparison between the numbers of crack branches made by water and viscous oil at the same distance.
Num
ber
of b
ranc
hes
0.0 0.4 0.8 1.2 1.6 2.0 2.4 2.8 3.2 3.6 4.0 4.4 4.8 5.2 5.6 6.0 6.4 6.8 7.2 7.6 8.0
Distance from the tip of the injection hole to the edge of the core (mm)
Water Oil
6
5
4
3
2
1